In such an inductive sifting of traditional beliefs, are there any that can be called knowledge? Is there one whose reliability cannot be successfully doubted? Not a single one, except the thinking process itself. I am certain that I am conscious. Even when in my universal doubt I say that nothing is certain, I am at least certain that I doubt. I am, therefore, contradicting my universal skepticism. To doubt is to think; in doubting, consciousness is asserting its existence. Skepticism is self-contradictory. An induction of our ideas reveals at least this one absolutely certain principle: I, as thinking, am. _Cogito ergo sum._ My own existence is an intuitive truth that accompanies every state of mind. This is the best known portion of Descartes’s philosophy, and perhaps it is in part to the Latin formula of it that it owes its widespread acceptance. It is criticised as trifling, even if it be true; and as reasoning in a circle. Yet it must be remembered that Descartes does not intend the _ergo sum_ (“therefore I am”) to be a conclusion of a syllogism of which _Cogito_ (“I think”) is the minor premise. This formula is not an inference, but an intuition, which is revealed by induction as the certain background of all knowledge. Three things are to be learned from this fundamental principle, said Descartes: (1) The first is that man has gained a criterion of truth. The characteristic of this principle that makes it reliable and certain is its clearness and distinctness. _Clearness and distinctness of ideas is the proof of their truth._ All true ideas will therefore have the mathematical and intuitive certainty that the idea of the existence of the self has. (2) The second lesson from this fundamental principle is that the existence of the soul is more certain than that of the body. The soul is more important and independent than the body. This is the subjective point of view of modern times. The modern man views the world as the representation or the creation of his thinking soul. (3) The third lesson from this principle concerns the nature of the soul. How long do you exist? As long as you think. (_Sum cogitans._) True existence is rational thinking, and God alone has it. Feelings and passions are obscure ideas.